Method of preparing dicyandiamide



Feb. 25, 1947. J, OSBORE 2,416,543

METHOD oF PREPARING DICYANDIAMIDE Filed Dec. 19, 1944 14447157? VHPORINVENTOR J O H N l. O S BO R N E ATTO RN EY Fsolvent. off'carbon dioxidein these prior processes, the disadvantages become still greater,forqthen the .use of gaseous-liquid reactions in the prepara.-

Patented Feb. 25, 1947 American Cyanamd Company, t N. Y., a corporationof Maine New York,

Application December 19, 1944, Serial No. 568,893

This invention relates to a process of preparing dicyandiamide, fromcrude calcium Icyanamide. i

The principal objectof the present invention is to preparedicyandiamide'by polymerizing cyanamide obtained from crude calciumcyanamide in an 'eilicient manner and in good yields and purity, whilesuppressing the formation of uridesirable by-products.

An important adjunct of the invention resides in facilitating contactbetween a gaseous prev'cipitant such as CO2 with calcium cyanamide suchas by use of .what may be termed a CO2 carrier, that is, by theuse ofamaterial which 'will react with CO2 and the resulting carbonate orbicarbonate go into solution, where it will then readily react withCaCNz. Such a situation makes possible a better contact of the gaseousprecipitation agent with the calcium cyanamide than if such agent wereused strictly in gaseous form, and consequently can result in a. savingof up to 25% of the CO2. Y

`7-In patent application Serial No. 568,892 it is .proposed carry out aprocess substantially iden- 'tical'with that of the present applicationemploying the alkali carbonates as the CO2 carrier. In Serial No.-568,894` guanylurea and guanylurea carbonate are em-ployed, and inSerial No. 568,895 the ethanolamines are employed.

Heretofore, as disclosed in the prior art, carbon dioxide has been usedto precipitate calcium carbonate from aqueous slurries of crude calciumcyanamide.l However, such processes involve, essentially, gaseous-liquidreactions. When "pure carbon dioxide. isused, such reactions necessitatebulky, expensive and usually complex mixing and diffusion apparatus 4inorder to com# minute the carbon dioxide bubbles and eifect'a bettercontact ofthe carbon dioxide and liquid When stack gases are used asasource tion of cyanamide and dicyandiamide necessi- .ftates theprocessing of large Volumes of gases.

This requires unusually large,y bulky equipment andnecessitatestreatment over extended time intervals. The operation ofsuch processes as disclosed inxthe priorfart is accordingly quite in'ecienaprincipally'because of the low solubility of carbon dioxide inwater. Furthermore, these priorprocesses also fail to overcome Vthedisadvantagesattendant upon the low solubility of lime and calciumcyanamide in the water used to make up the aqueous slurry. i

The present process involves a novel method of facilitating the reactionbetween carbon dioxide and crude calcium cyanamide slurries. It relatesto asimple process for bringing about the better solution of carbondioxide inthe water forming the slurry., Thus, it has `been found thatvbythe Vacldition'of a suitable Water soluble compound capable ofcarrying carbon dioxide in combination there is'efected what isessentially a marked increase in the solubility ofrcarbon dioxide. Fur-,thermorethe reaction of CO2 and calciumcyanamide completedxmoreefficiently. ThisQis due to the fact that Athe :process is effected'asfaliquid to liquidrathenthan as a gasto liquid type of reaction, Thesolubilizing of the carbon dioxide converts the latter into a morereadily available condition, diiused throughout the liquid, and henceeiiects the reaction as one closely approximating an ideal liquid toliquid type of contact. l Y i f These novel features in the process ofthe present invention make possible theuse of smaller,

The present processalso shortens the ntime necessary 'to complete thereaction of carbon dioxide with thelime and the CaCNz. At the same timeit makes possible the, use of, lower pressures ofV carbonv dioxidewhileeffecting a greater concentration-oi carbon dioxide in solution. Thisresultsv in a more `eilicient process and a more uniform quality-cfproduct. Among the `compounds capable of rei acting-in this, manner arethe following: `the alkali metal carbonates, such as sodium andpotassillmj carbonate, guandine and guanylurea, both as `such and astheir carbonates, the ethanolamines such as the mono, di andvtrijformsand also the alkyl amides suchas the mono and poly methyl andethyl amines. f

More specifically, in accordancewith one'emvloodiment of this invention,it is proposed that anV `passed into the resultant slurry. It will befound duction of NH3, urea and thelike.

may bei prepared in an atmosphere of carbon di. voxide underVsuperatmospheric pressure in an that the alkyl amine serves to dissolveCO2 which reacts With calcium cyanamide and accelerates the formationvof calcium carbonate. The alkyl amine reacts with the Co2 to form thecorresponding carbonate, whichy in turn reacts with the calcium of thecrude calcium cyanamide. It also reacts with the lime which is presentin the slurry as calcium hydroxide. This reforms the alkyl amine and aprecipitate of calcium carbonate. bon Vdioxide at a very rapid'rate,reforming alkyl amine (zailooriz/ate which then `reacts with the Thealkyl amine absorbs additional car-` calcium acid cyanamid'e in theextraction liquor.`

This yields calcium "carbonate as a precipitate and free cyanamide. Thissolution may be then concentrated as by evaporation, and anycyanamide'therein not already polymerized, polymer-,vy Y* izes to formdicyandiamide Which is readily extracted by filtration. l

It is to be noted that theuse ofY the process of;

tered. When the mother liquor attains itsV optimum concentration ofcyanamide or dicyandiamide it is withdrawn through the by-pass forfurther treatment. After a predetermined V num-- ber of recycling stepsincluding the reslurrying of the filter cake, the lter cake remaining asa sludge in the final filtration step in Washed with a small amount ofWash liquor in order to remove the greater portion of the'entrainedcarbon dioxide carrier left therein. This wash liquor is recycled toform an aliquot part of a new batch or slurry of calcium cyanamide. Theysludge remaining inthelter is removed from the system by scraping thelter-plates clean ofthe spent sludge. vA new slurry is then prepared asabove described, the cycling and recycling steps are carried out ,Yagain terminating in a Afinal filtration and removal from the extractioncycle of the mother "liquor containing its-optimum concentration of thisinvention not only makes possible a greaterconcentration of carbondioxide inthe slurry but; also results in the presence of carbon dioxidein a more readily reacting form than the usual gase-:V

ous state. This is dueto the addition to thev slurryof the abovementioned compounds. lAll of these compounds have a high aflinity forcarbon dioxide which is held in combination therewith. Hence these arecapable of serving as carriers of carbon dioxide held in combination'and readily 'available forfurther reaction as with the calcium` of theslurry. These compounds make possib-le amore complete absorption andsolution of carbon dioxide/in -the mother liquor thancould be effectedif the' carbon dioxide were bubbled through a slurry containing no suchcarrier compounds.M Furthermore with these vcompounds present, thecarbon dioxide need be under little' `or no pressure in order to obtainahigh concenltration of available carbon dioxide.v Heretofore equallyhigh concentrations of carboni dioxide could bev attained only by usingcarbon dioxide under several atmospheres .lpressure`. Because o'f thelow pressure solubilizing 'step characteristic of theprocessof thisinvention, it is unnecessary to carry outl the process in such expensiveair- Adesirable yields and avoid excessive decomposi tion due to sidereactions which result in the pro- The slurry autoclave or-,alternatively, carbon dioxide is bubbled` through the slurry atatmospheric pressure.v 'AThe-ireactantsare mixed, preferably, for

*an additional time interval ina second tank to` increase the extractionof the calcium cyanamide after which the slurry is filtered.v In orderto iny crease the concentration of nitrogen-present in the form ofcyanamide and later as dicyandiamide, the mother liquor forming theclear ltrate is recycled a number of times and reslurried withadditional portions of crude calcium-cyanamide, the

y combined recycled mixture being then finally fil-` vcvinarriide ordicyandiamide. This optimum concentration may be one either low incynamide and-high in dicyandiamide or the reverse. The former situationis a highly desirable one inasmuch as by building up the content ofdicyandiamide in the mother liquor, that is, by polymerization of theY,cyanarnide to 'dicyandiamida as the Vformer is freed from. calciumcyanamide, the liquor going through the by-pass will. contain more and'more dicyandiamide making 'evaporationat this stage unnecessary; Thatdicyandiamide can be built up in the mother liquor will be apparent whenit is considered that the liquor at. this stage is of sufficiently highalkalinityfand rtemperature to facilitate polymerization of thecyanamide to dicyandiamide.- This situation will loev more fullydiscussed hereinafter.

"In addition 'to the above .batch process, the process can be readilyeffected by Vcontinuously drawing off a fractional part ofthe motherliquor or concentrate rthrough/the icy-pass for further specialtreatment. The remaining portion is returned o-r recycled tothe rst tankWhere more crude calcium cyanamide and carbon dioxide as well as washliquor and mother liquor from other sources, as shown, can be` added tomaintain the recycling volume constant.

The mother liquor withdrawn through the bypass may be concentrated in anevaporator, preflerably by low temperature vacuum distillation, toremove the excess water. The carbon dioxide held in combination by rthecarrier is simultaneously evolved with the result that the alkalinity ofthe solution maybe increased to that of a solution having a pHfofvlL) orthereabouts, where `upon practically all of the` cyanamide polymerizesto dicyandiamide. Thewdicyandiamide,

containing Vperhaps alsmall amount.' of cyanamide in thev entrainedliquor, is vreadily sepf arated out by cooling `and filtering theVS0111.- tion. The mother liquor forming the ltrate is recycled as shownto form a new slurry. If contaminatedunduly with decomposition products.

such as urea for example, it may bepreferable to Y discard this ltrateperiodically.

Whenrvacuum evaporation of vthe by-passed mother liquor is conductedfata low temperature, i. e., about 30 C. to.35.C., most of the cyanamidepresent polymerizes to form dicyandiamide due to -thehigh alkalinity ofthe-solution.y Any remaining free cyanamide.subsequently'polymerizesonstorage. However, if complete conversion of the.

cyanamide to dicyandiamide is desired immediately, the evaporation steplmay beA omitted, particularly if the solution is stronger than about 10or 12%. Thus, if the solution has a concen agregue forth. the followingis an important consideration.

It would, of course, be desirable to voperatgthe cycle under suchconditions thaty most, if not all, of the cyanamide Went directly todicyandiamide Yreasonably as fast as the cyanamide Was freed,maintaining enough water in the system to keep the dicyandiamide insolution so that when the final liquor reached the evaporator the dicy-4andiamide could be recovered therefrom eitherby evaporation or chillingor both.

`The above may be readily accomplished by maintaining the pH of thesystem ofv'7.0 or above. While high temperatures are desirable in orderto increase the rate of polymerization of cyanamide todicyandiamide,yet` the temperature is not as important as the pH.

It IWill be apparent thatvas the calcium eyanamide is fed to the mixingtank the pH has a tend-y ency to rise, whereas when the CO2` is fed vtothe vmixing tank the pH has a tendency to fall. Thus, .it is asimplematter, by judicious controltof these two reactants, tomaintainthe pH between 7.0 and 12.0 so as t0 maintain desirable cyanamidepolymerizing conditions. This Vcan be very simplymaccomplished bykeeping the calcium cyanamidefeed ahead of the carbon dioxide feed. The

temperature of the extraction cycle may be maintained at from 30l C. toboiling, the exact figure depending upon the existing pH. For a given pHwithin the desired range,fthe higher the temconverted to dicyandiamideat a pH of 9.6While at higher temperature such as 80 C. the optimum pHis about 9. Atboiling Vtemperatures the pH may drop as low as 3 Whilestill obtaining polymerization of cyanamide to dicyandiamicle` at goodrates.

The increase in temperature to increasethe Thus the sensible heat of*indicated;y .l 'Only @Small ...thesystem by failure towash lthe'firstfilter cake.

. 6 lis'I'haintainedno* lower than7i0 and. preferably from 8.0 to11'or's'lightly higher with a` tempera- 'ture of 'from'30 Cftoboilingrlirise' lnltemperay ture for any givenrpH Vvvillg'of course,'increas'ethespeed of polymerization*and.v as the temperature rises the pH maybedecreased toward 7.0 while ,'stillinaintainirfg the; samepolymeriz'ationrate. kUnderthese circumstances, it is'l desirablel to maintain'sil'.flcient 4Waterin the system to v'keep all of the dicyandiamideformed in solution plus a' slight excess so that'there will be no lossof jdicyandi'amide'values during the normal andfexpectedtemperature dropof 4the motherliquor'as it passes through the fiilter. f

` k'Ihus'iitisfentirely -possible to maintainV the 'cycle'under suchconditions'of pH and temperature .Y that there Willbe `little or nocyanamide vgoing through the'byf-Vp'ass Abut on the contrary thisvby-passed motherliquor will be almost exclusively a desirably highconcentration of aqueous dicyandiamide solution containing a mini- .mum'quantityv of... undesirable'. decomposition products such asurea. Underfthes'ejzcircum- .thamother liquor and recovered assuchl The .motherliquor fromthe dicyandiamide recovery systemnnaythen be returned to thecycle as.

amount Vof the carrier is i lost from Another small portion of thecarrier mayzbe lost by failure to Wash thelnal filter cake.' Hov'v'-ever, by flushing these filter "cakes with Wash water the-carriertherein is -easilyremoved Thus, the ,entrained carrier may .be removedfrom the final product by washingit out ofthe filter cake withaI-smallvportion oi?` vWater andsimultaneously v`leaving the relativelyinsoluble dicyandiamide on the filter.` ventire'supply of thecarrier foreach extraction.

Hence it is unnecessaryrto renew the Instead, the addition ofVr vasmall'portion of the carrieror `an, aqueous Vsolution thereof to the.recycled mother liquor at intervals duringvmt/he 1 processor after eachuextractiongcycle is ample to increase. the; concentration of. thecarrier 'i in l theamother liquor sufiiciently forefective re use uponfurther addition of carbon dioxide.

Y.tionand/orconversion cycle, 'the solids arebeing continuously removed.Thus any desirable lcon- .Due to..thejinclusion of a lter in thevextraccentration of solubles up to and-just short of sat- -11ration inthe liquor for any. desirable Working A .temperature may be had withoutmaking the recycle magma unhandlable due to solids. .3. V TheJ reactionsinyolvedmay `be briefly repre- '.1 sented 'by'. the following equationswhen. sodium carbonate-is used as the carbon dioxide carrier:

Na2CO3+COz+H2O 2NaHCO3 i vThe sodium bicarbonate formed serves as ahighthe temperature in the mixing tank tends to rise`- to undue heights,this kcan be readily controlled" tothe extent of about 12%, the otherconstitu` '"l-y-accessible source of carbonv dioxide.. and reacts vviththe crude calciumcyanamideto precipitate 'calciumcarbonate as follows:f. f

. ents being in the nature of diluents, a large volv "ume of gas may bepassed through the magma anamide may be released from a calcium:cyana-4*mide slurryin the presence of CO2 where the pH 'A iitinue thevprecipitation of more calcium. 1H v'The corresponding yequations occurywhen :S'odiuni carbonate is thus reformed and is in condition to reactimmediately with the incoming carbon dioxide to give the bicarbonateandy contassiuml carbonate,` guanidine,4 guanylurea, either laldde'das'such orgused as the carbonate, ethanolamine such asmono,"'diortri ethanolamlne, mono,

Y 100e-parteci.waten2 .cyanamide and 10 parts ormone' methyl amine.bxltration 'tates out of solution'and is readily'separated by y'to'form Y .and give the,y corresponding. bicarbonate rsolution havinga pHof about 8.4. The resulting slurry is mixed further in anothermixingvkettle and rthen filtered. The filtrate is evaporated at a temperaturebelowl 35? C., cooledandthe crystalline precipitate,predominantly;.dicyandiamide cori.-

taminated with such smallamountsofcyanamide as, is present in.theentrained liquor, recovered Example 2.Yv

rExample l' is Vrepeatedcarrying out the evapo-V rationfstep ataw'higher temperat'ure.whichv results ini practically puredicyandiamide' being obtained. Thus, when 'evaporation` isveiecte'd atahigher l 'temperature in the ran'gejof about; 60fto 80 v C., "or higherthe cyanamide V"polymerizes yielding` 'practically 100% dicyandiamidewhich precipi-1 coolingl and ltering. The ltrate comprising mother.liquor containinglCO; carrier'isrecycled a new batch or slurryuoicalcium cyan- Exampley l 3 100 parts of crudecalciuntcyanamide.aregrad-1 ter containing` some carrierandmother liquor containing the aminecarbonatefrom a'previous batch extraction.`v The'slurryis fortified withenough mono methyl amine to bring its equivallentv up to 1'0 parts and50 parts of carbon clioxide are simultaneously suppliedwhile maintainingthe pHat 8.4andkeeping the temperature below 35C.` The resulting-slurryis passed 'to another. mixing kettleandthen .lteredz About 275 partsof*` ltrate'areobtained containing from 110%to 12% of cyanamide-ifThe-sludge onthe filter is washed and the Wash Water recycled-t0formpart of a new .batch.1 Duefto the lowcon-AV I centration of` valuesin lthel lfiltrate, thev .latter is "furtherevaporated under vacuum andbelow: 35` until crystals beginjto.V formwhereuporr the liquidischi-lled and' filtered. Theyieldis predominantly dicyandiamide, thecyanamidewhich rst formed having vpollymerizedto dicyandiarnide, becauseof the increased.pH-.1 The filtrateis rei" v cycled to form-fthemother'. liquorfor the. next baten M 1 Ervemlzl lattenisrelativelyinsoluble vin the cooled mother liquor, tha-dicyandiamide is readilyseparatedby filtering thecrystalline precipitate..u The ltrate isthenrecycled to lform another slurry of calcium cyanamidev for .subsequentextraction. The extraction step of the process the preceding examples ispreferably carried' out at a pH of approximately 8.4. Various changesmay, howeven. be made in the particular steps,

thusiorexamplaif stack gases are used asthe source v`of. carbondioxideit may be preferable to strip the CO2 from. the stack gases with an.aqueous solution of va carrier'and thereafter vadmix the carriersolution with the slurry, thereby pre- Avbatcnsimuarqrc that.described-@Examine 3 is subjected. tothesame series of- `steps.toAobtain a. corresponding 275,part filtrate.. In. order-to obtainr apractically.v complete. .conversion.v kto -dicyandiamidathe solution ofcyanamide .comprising the 275'part ltrate is 'evaporated'at fa tem`perature ofabout 70C., althoughother texmtrera-` turescan also beused-upr` to boiling. Ahighly alkaline .solutionis ormevdrby theevolution, oft

containing., the carrier.' n having api-I between isherlemperatres abovegiven.v `are usedto .ecce ratefthe polymerization CO2 from the vsolutioThis. results., `in La sol'Y entre @werden acercarme.[Sinatrafcipitating CaCOathe whole mixture being led rectly: into thefirst mixingtank. i n A .featureof thisinvention isthe relatively com'- pletecontrol of theprocess and the relatively rapid initiation ofthereaction-obtained by the use of an alkyl amine or a similar CO2carrier.

When the above examples are repeated without the use of a CO2carrier'substantially longer periods of time are required ,to eiect thecalcium precipitation. Also due to the lesser speed.. of absorption ofCO2 without'the carrier, proportionately more` CO2 is requiredbecausemore CO2 passes through the slurry unabsorbed.`

In the examples given, NazCOs, K2CO3; guanidinet or guanylurea,.either'as. such VOras their carbonates, or an ethanolamineY orcli,'tri,f methyl ormono, di or tri ethyl amine can be used in place of.the .mono Ymethyl amine.. `In casethe other alkyl amines mentioned. areusedA the followingnumber of partsthereof should be present:

f Y Parts Dimethyl amine '15 Trimethyl amine 20 Monoethylamine 15Diethyl amine- 25 Triethyl amine", 35`

In each instance dilute solutionsof the respective carbon dioxidecarriers of from 3% to 5% or even l10% strength-.fall in a desirablerange.

What is claimed:

l. A method of preparing dicyandiamide which includes slurrying calciumcyanamide in thepresence ofCOz anda substance-chosen from the groupconsistingy of mono, di, andhtrimethylamine and menor, dif, andtrifethylarnine,

in sufficient Water. toV maintain all the dicyandiamide formed insolution, VInaintainng a pH bctwcen, '7.0V and 12.0 and-a temperaturebetween Y 30 C. and boiling, whereby calciumvrcyanamide; v`is convertedto calcium carbonate and dicyandiamide, separating the calcium carbonatefrom-.the mother liquor and recovering the dicyandiamide from `theVAlatter.

2. A method of preparing dicyandiamide which vincludes slurry-ingcalciumcyanamide in the presenceof CO2-and a. substance chosen from the.

group consisting ci mon-, di, and'v tri-methyla-l 1 mineVv andv rnono,di, andv tri-ethylamine, in

suliicient water tomaintainallthe dicyandiamide formedinsolution,maintaining a. pH between 777.0 and 12.0 anda temperature between 30 C.and

boiling, separating out the calcium solids, recirculating themotherliquor therefroml with fresh vadditions'of calcium cyanamide and CO2,filterin gout the, solids With each recirculation,until the motherliquorY from the solids removal is` so .short of saturation withdicyandiamide, and removing the latter therefrom.

' 3. A 'rn'ethod oflpreparing dicyandiamide which `includesslurryingcalcium cyanamidein the pres--V described ence of CO2 andmonomethylamine, with sumcient water to maintain all the-dicyandiamideformed in solution, maintaining the pI-I between 7.0 andv 12.0 and thetemperature4 between 30 C. and boiling, ltering out the solids,recirculating the mother liquor with fresh additions of calcium cyanamdeand CO2 until the mother liquor from the solids removal is just short ofsaturation with dicyandiamide, and removing the latter there.

UNITED STATES PATENTS Number n Name Y Date 2,337,488 Osborne Dec. 21,'1943 REFERENCES CITED The following references are of record in the fueof this patent:

0 Re.1s,958 Bottoms s sept.\26, y1933

